Fructose (levulose) is generally regarded to be substantially sweeter than glucose (dextrose) and consequently is more desirable and commands a higher price in the marketplace for that reason. However, glucose is more easily available from inexpensive sources. A practical and economical process for the conversion of glucose to fructose is therefore desirable.
There are several enzymes that convert glucose to fructose directly. Shieh, Lee and Donnelly (U.S. Pat. No. 3,834,988) have disclosed a method for isomerizing aldoses, such as glucose and xylose, to the corresponding ketoses by using enzymes produced by microorganisms of the Actinoplanes genus. Their process, however, does not achieve its best results in the absence of magnesium ion and cobaltous ion. Magnesium and cobalt salts are added to the glucose-containing substrate in the Shieh et al process to activate and heat-stabilize the isomerase.
Glucose syrups are produced from corn starch by acid conversion, acid-enzyme conversion or enzyme-enzyme conversion. Pure glucose can be produced by crystallization of glucose from a glucose syrup.
If pure glucose is used or if the glucose syrup from the aforesaid conversions is refined by filtration or filtration-carbon treatment or filtration-carbon treatment-ion exchange, a glucose syrup results which, when used as a substrate by glucose isomerizing enzymes, requires addition of magnesium and cobalt salts to produce good yields of fructose.
These fructose containing syrups must then be ion exchanged to remove the magnesium and cobalt salts from the syrup. This additional ion exchange refining step adds to the cost of the syrup product.
In the conventional process, when corn starch is hydrolyzed by acid or acid-enzyme or enzyme-enzyme processes, a material which is generally called "mud" is released from the starch matrix as a result of the gelatinization and subsequent thinning and/or saccharification. This mud is the crude water insoluble fat containing material resulting from hydrolysis.
We have discovered that if this mud is allowed to remain in the glucose-containing substrate during treatment with glucose isomerizing enzyme, the activation and stability of the glucose isomerizing enzyme are enhanced without the necessity of adding cobalt and/or magnesium salts during the isomerization reaction. If cobalt and/or magnesium salts are added during the process, they must be removed by ion exchange to produce the most satisfactory final syrup.
The glucose isomerizing activator factor or component in the mud is one or more of the fatty acid constituents. We have discovered that we can extract the mud with a fatty acid solvent and by adding the fatty acid solvent soluble fraction to a purified high glucose substrate we get an enhanced isomerization effect.
We have also discovered that when the mud remains in the substrate during the conversion of glucose to fructose by isomerizing enzyme, some of the calcium ion normally present in the substrate is complexed and is thus not available for interaction with the isomerizing enzyme. Calcium ion is usually considered to be detrimental to glucose isomerizing enzymes.
Accordingly, one of the principal objects of this invention is to provide a process for converting glucose to fructose without adding cobalt and/or magnesium salts to the substrate. Another principal object is to provide a process for producing fructose which does not require the final glucose-fructose syrup to be ion-exchanged after isomerization.
Still another object is to provide a process for producing fructose syrups from a high glucose substrate by conversion with a glucose isomerase in which the process can tolerate higher levels of calcium ion without inactivating the enzyme than conventional isomerization methods. These and other objects and advantages will become apparent hereinafter.